Method for manufacturing MEMS structures

ABSTRACT

A method for forming a free standing micro-structural member including providing a substrate; blanket depositing a first sacrificial resist layer over the substrate; exposing and developing the first sacrificial resist layer to form a first resist portion; subjecting the first resist portion to at least a hard bake process to form the first resist portion having a predetermined first smaller volume compared to a desired final resist portion volume; blanket depositing at least a second sacrificial resist layer followed by exposure, development and the at least a hard bake process to form the final resist portion volume; and, depositing at least one structural material layer over the final resist portion.

FIELD OF THE INVENTION

This invention generally relates to manufacturingmicro-electro-mechanical systems (MEMS) and more particularly a methodof for manufacturing MEMS structural components according to sacrificialresist patterning methods.

BACKGROUND OF THE INVENTION

Increasingly, there is a demand for the fabrication of 3-dimensionalmicron-scale components for micro-electro-mechanical systems (MEMS).Micro-electro-mechanical devices include structures of generallyconventional shape and function, e.g., beams, posts levers, wheels, andthe like, but of a size that is on the scale of hundreds of microns orsmaller. As the general name implies, MEMS often incorporateelectro-mechanical elements as sensors and/or actuators includingoptical components such as electro-mechanical mirrors and the like.

In one approach to fabricating MEMS structural components a3-dimensional sacrificial resist mold is formed on a substrate fordepositing a structural material. Generally, micro-lithographictechniques conventional in micro-integrated circuit fabrication havebeen used to form shaped structures on substrates. The adaptation ofsemiconductor manufacturing techniques has also been favored becausesilicon has been found to be a useful material for making MEMS.

In addition, other structural materials, such as metals, oxides andnitrides have been used for forming MEMS structural components.Generally, the approach includes successive steps of applying asacrificial resist layer, patterning the resist layer, and forming astructure corresponding to the pattern. The MEMS structures may beformed by either etching a substrate according to the patterned resistlayer or by depositing a structural material over the patternedsacrificial resist layer to form a 3-dimensional structure on thesubstrate surface. Successive stages of patterned deposition and etchingmay be used to form arrays of larger 3-dimensional MEMS structures.

A particular problem encountered in MEMS manufacture, which is not sooften experienced in fabrication of semiconductor devices is the need toprovide vertical dimensions and aspect ratios with greater tolerancesthan those commonly demanded in the fabrication of semiconductordevices. One problem in using sacrificial resists is the tendency of thesacrificial resist to shrink in volume upon curing the resist, includinga hard bake process following exposure and development of the resist. Asa result, the mass volume of the patterned resist is reduced, alteringthe critical dimensions of the patterned resist in unpredictable anduncontrollable ways and compromising the critical dimensions of thesubsequently formed MEMS structure.

For example, referring to FIG. 1A, is shown a patterned resist layerportion 12 formed over substrate 10. Referring to FIG. 1B, is shown thepatterned resist layer portion 12 following a curing process including ahard bake where sidewall portions e.g., 12B are recessed due to resistshrinkage. Referring to FIG. 1C, subsequent deposition of the structuralforming layer 14 results in a thinned structural layer e.g., 14B alongthe sidewalls, resulting in a deformed structural portion compromisingdesign constraints including mechanically weakening the overallstructure.

Accordingly, there is a need in the MEMS fabrication art for an improvedmethod to form structural components with improved dimensional accuracyand mechanical integrity including fabricating free-standing structureswith high aspect ratios.

It is therefore an object of the invention to provide in the MEMSfabrication art an improved method to form structural components withimproved dimensional accuracy and mechanical integrity includingfabricating free-standing structures with high aspect ratios, inaddition to overcoming other shortcomings of the prior art.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the present invention provides a method for forming a freestanding micro-structural member.

In a first embodiment the method includes providing a substrate; blanketdepositing a first sacrificial resist layer over the substrate; exposingand developing the first sacrificial resist layer to form a first resistportion; subjecting the first resist portion to at least a hard bakeprocess to form the first resist portion having a predetermined firstsmaller volume compared to a desired final resist portion volume;blanket depositing at least a second sacrificial resist layer followedby exposure, development and the at least a hard bake process to formthe final resist portion volume; and, depositing at least one structuralmaterial layer over the final resist portion.

These and other embodiments, aspects and features of the invention willbe better understood from a detailed description of the preferredembodiments of the invention which are further described below inconjunction with the accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A–1C are representational cross sectional views of a portion of aMEMS structure at stages of manufacture according to the prior art.

FIGS. 2A–2E are representational cross sectional views of a portion ofan exemplary are representational cross sectional views of a portion ofa MEMS structure at stages of manufacture according to an embodiment ofthe present invention.

FIG. 3 is a process flow diagram including several embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the method of the present invention is explained by exemplaryreference to a rectangularly shaped structure including sidewallportions, it will be appreciated that the method of the presentinvention may be adapted to any shaped MEMS structural component wheresacrificial resist layers may be advantageously used as a sacrificialmold for depositing a structural material thereover. It willadditionally be appreciated that successive stages of the method of thepresent invention may be repeated to form a larger 3-dimensionalstructure including an array of 3-dimensional structures.

Referring to FIG. 2A, a first sacrificial resist layer is deposited andpatterned by conventional means to form a patterned first sacrificialresist layer portion 22 over substrate 20. The first sacrificial resistlayer portion 22 is formed at a predetermined smaller volume dimensionthan the desired final resist portion volume dimension for subsequentstructural material deposition thereover.

For example, the first sacrificial resist layer portion 22 is depositedby a conventional spin coating process, for example including anyconventional photoresist including DUV and DNQ novolak I-linephotoresist. Following spin-coating, the resist is subjected to a softbake at a temperature range of 85° C. to about 125° C. to drive off aportion of the solvents and impart dimensional stability to thephotoresist. Following the soft bake, the photoresist layer is alignedand exposed through a mask by conventional methods. Following exposure,the resist is preferably subjected to a post-exposure bake (PEB) tofurther drive off solvents to leave less than about 10% solvents in theresist. For deep ultraviolet (DUV) resists the PEB process is criticalin order to catalyze a chemical reaction and make the resist soluble inthe developer. Preferably, the PEB process is carried out from about 5°C. to about 20° C. higher than the soft-bake process.

Following the PEB process, the resist is developed by conventionalprocess, for example using conventional tetra-methyl-ammonium-hydroxide(TMAH) containing developer formulations to leave the patterned firstsacrificial resist layer portion 22.

Still referring to FIG. 2A, preferably, the first sacrificial resistlayer portion 22 is patterned such that following development it isformed with a volume smaller than that of the desired final resistportion volume (thickness, width and depth dimension) from about 5% toabout 50% smaller in volume, more preferably from about 10% to about 33%smaller in volume. For example in the case of a rectangular firstsacrificial resist layer portion e.g., 22 the width (and depth) definingsidewall portions are formed at a smaller dimension difference value S1compared to the final desired width (or depth) dimension S, S1 having avalue of about ½ compared to the smaller thickness dimension value T1,T1 being the difference compared to the final desired thicknessdimension T. For example, if the smaller dimension value T1 compared tothe desired final dimension T is smaller by about 1 micron, the sidewallportions have a width (or depth) formed at a smaller dimensiondifference value, S1, smaller by about 0.5 microns compared to thedesired final dimension S, such that the total smaller dimensiondifference value (2×S1) for the width and depth portions is about thesame as the smaller dimension difference value T1, e.g., about 1 micron.

It will be appreciated that the smaller dimensional difference valuesfor S1 and T1 will vary depending on the width or depth of the resistlayer portion 22 and the degree of shrinkage expected following a hardbake process, for example from about 1% to about 10% of the width ordepth dimension. In addition, the desired tolerances of the structuralmember to be formed by structural material deposition over the resistlayer portion e.g., 22 following a UV/hard bake process according to anembodiment of the invention, will determine the size of the smallerdimension values as well as determining whether more than one additionalsacrificial resist layer deposited over the resist layer portion 22 willbe necessary to achieve desired tolerances as explained below.

Referring to FIG. 2B, following the developing process, the firstsacrificial resist layer portion 22 is subjected to at least a hard bakeprocess, more preferably both a UV exposure, preferably deep UV (e.g.,less than about 350 nm), and a subsequent or simultaneous hard bakeprocess from about 250° C. to about 350° C. For example, exposure to UV,preferably deep UV, promotes polymeric cross-linking reactions at thesurface of the resist, forming a hardened resist shell (surface portion)thereby preventing distortion of at least the UV exposed portion whileallowing a higher hard bake temperature without causing flowing anddistortion of the resist portion, e.g., upper surface portion 22A. Thehigher temperature tolerance to resist flow is important in subsequentstructural material layer deposition over the sacrificial resist layerportions as deposition temperatures can reach up to about 200° C.

Nevertheless, as seen in FIG. 2B, the sidewall portions e.g., 22B, ofthe first sacrificial resist layer portion 22 tend to ‘cave in’ due tovolume (mass) loss (shrinkage) caused by volatization of a remainingportion of solvents present in the resist and limited hardened shelldevelopment at the sidewall portions due to limited deep UV exposure.

In one embodiment, the hard bake (thermal heating) step is carried outfollowing a deep UV exposure step, or simultaneously with at least aportion of the deep UV exposure step step. The s application ofpolymeric cross-linking inducing irradiant energy (e.g. deep UVirradiation) during at least a portion of the heating step, for example,initiating UV irradiation either prior to or following initiating of theheating step, preferably prior to initiation of the heating step can beoptimized to allow partial outgassing prior to formation of the hardenedresist shell at the resist surface thereby preventing undesiredlocalized swelling of resist portions or bursting of resist bubblesformed during resist outgassing. Optionally, the resist temperature maybe ramped up to a baking temperature at about 10° C./min to about 30°C./min, preferably a baking temperatures from about 250° C. to about350° C. following or at least partially simultaneously with irradiationof the resist with deep UV light. Alternatively, the resist portion maybe first subjected to deep UV irradiation for a predetermined periodfollowed by heating (hard baking) the resist layer at the bakingtemperature, for example from about 10 minutes to about 60 minutes.

Referring to FIG. 2C, following the UV/hard bake process, at least asecond sacrificial resist layer portion 24 is deposited over the firstsacrificial resist layer portion 22. The same sequence of processes isthen followed as outlined for the first sacrificial resist layer portion22, e.g., soft bake, exposure, PEB, development, and UV/hard bakeprocesses. For example in one embodiment, the second sacrificial resistlayer portion 24 is deposited to a thickness such that following theUV/hard bake processes, the volume or dimensional values e.g., S2 and T2of the second sacrificial resist layer portion 24 together with thefirst sacrificial resist layer portion 22 together make up a desiredfinal resist portion dimension or volume.

In another embodiment, the second sacrificial resist layer portion 24may be formed such that following the same processes as for the firstsacrificial resist layer portion 24, e.g., soft bake, exposure, PEB,development, and UV/hard bake, the resist portion has dimensions thatremain smaller than the desired final resist portion dimensions orvolume, for example making up about ½ of the difference between thefinal desired dimensions and the first sacrificial resist layer portion22 dimensions. Thereafter, the process is repeated with deposition of asubsequent, e.g., third sacrificial resist layer portion (not shown)with the same processing steps to make up a final desired resist portiondimension or volume following a UV/hard bake process.

Since the formation of a hardened shell at the resist portion surfacesfrom UV exposure may be not fully effective on the sidewall portions dueto light shadowing effects of adjacent resist portions (not shown), adegree of resist shrinkage along the sidewall portions of thesubsequently deposited resist layer portions e.g., 24 will occurfollowing a hard bake process, for example, from about 1% to about 7%.Depending on the thickness of a subsequent sacrificial resist layer tobe deposited to approach a desired final resist portion volume and thedesired dimensional tolerances desired for the subsequent structuralmaterial layer to be deposited, the subsequent sacrificial resist layer(e.g., second sacrificial resist layer e.g., 24) may desirably bedeposited to a thickness with a dimensional volume smaller than thedesired final resist portion dimensional volume. Following a hard bakeprocess, a subsequent sacrificial resist layer (e.g., third sacrificialresist layer) is deposited over the second sacrificial layer portione.g., 24 and subjected to the same processes, e.g., soft bake, exposure,PEB, development, and UV/hard bake, to achieve a desired final resistportion dimension or volume.

Referring to FIG. 2D, following formation of the resist layer portionse.g., 22 and 24, to reach a desired final resist portion volume, ablanket deposition process is carried out to deposit a structuralmaterial layer 26 over the last deposited resist layer e.g., 24. Forexample the structural material layer 26 may be any structural materialused for MEMS structures including single or multiple layers of metals,metal nitrides, refractory metals, refractory metal nitrides, oxides,carbides, and piezo-electric oxides such as PZT, a solid solution oflead titanate and lead zirconate e.g., (Pb (Ti, Zr)O₃. For example, thestructural material layer 26 is preferably deposited at a temperaturelower than a softening point of the resist layer portion, for exampleabout 210° C. For example, a low temperature CVD or PECVD processesusing organo-metallic precursors or physical vapor deposition (PVD)processes process may be used where the deposition rate is controlled tokeep the heating of the photoresist layer below a softening point. Inaddition, an electrochemical plating (ECP) process preceded by PVDdeposition of a seed layer may suitably be used to deposit a metal. Thethickness of the structural material layer 26 will of course depend onthe structure formed, for example considerations of strength, stiffnessand resonant frequency will typically dictate the desired thickness ofthe structural material layer 26.

Referring to FIG. 2E, following deposition of the structural materiallayer 26, an opening may be formed to expose a portion of the resistlayer portion e.g., 22 And 24, for example the substrate 20, e.g.,silicon, may be etched through from the backside to form a backsideopening portion e.g., 28. A resist removal process, preferably an oxygencontaining ashing process is then used to remove the resist layerportions e.g., 22 and 24 to leave a free standing structural member 26.For example an oxygen ashing process may be used alone or in conjunctionwith a conventional wet stripping process as is known in the art ofintegrated circuit manufacturing.

Referring to FIG. 3 is a process flow diagram including severalembodiments of the method of the present invention. In process 301, afirst sacrificial resist layer is blanket deposited over a substrate andpatterned to form a first resist volume portion having a smallerdimension (volume) than a final desired dimension (volume). In process303, the first resist volume portion is subjected to irradiant energy(e.g., deep UV) to induce polymeric cross-linking and thermal heating(hard bake) inducing resist volume shrinkage and hardening. In process305, at least one additional (subsequent) sacrificial resist layer isblanket deposited over the first resist volume portion and patterned toapproach or reach a predetermined (final) total resist volume portion.In process 307 the subsequent resist volume portion is subjected to asubsequent irradiant energy (e.g., deep UV)/thermal heating (hard bake)step to achieve a predetermined desired resist volume portion(dimension). In process 309, at least one layer of a structural materialis blanket deposited over the resist layer portion. In process 311, theresist portion is removed to leave a free-standing structural materialportion.

Thus, a method has been presented for forming free-standing structuralportions to desired dimensional constraints by using at least twosacrificial resist layers to form a resist portion (mold) for subsequentstructural material deposition thereover thereby reducing thedimensional variations in the structural portions due to resistshrinkage in a resist patterning and curing process. As a result,free-standing structures, including MEMS structures may be formed totighter dimensional tolerances with improved structural and mechanicalintegrity.

The preferred embodiments, aspects, and features of the invention havingbeen described, it will be apparent to those skilled in the art thatnumerous variations, modifications, and substitutions may be madewithout departing from the spirit of the invention as disclosed andfurther claimed below.

1. A method for forming a free standing micro-structural membercomprising the steps of: providing a substrate; forming a firstsacrificial resist layer over the substrate; patterning the firstsacrificial resist layer to form a first resist portion; subjecting thefirst resist portion to at least a first hard bake process to form thefirst resist portion having a first volume; forming at least a secondsacrificial resist layer on a top surface of said first resist portionfollowed by patterning and conducting at least a second hard bakeprocess to form a final resist portion having a final volume; depositingat least one structural material layer over the final resist portion;and removing said final resist portion.
 2. The method of claim 1,wherein the at least a first hard bake process further comprises one ofa prior exposure or at least partially simultaneous exposure topolymeric cross-linking inducing radiant energy.
 3. The method of claim2, wherein the radiant energy comprises ultraviolet light having awavelength of less than about 350 nm.
 4. The method of claim 3, whereinthe ultraviolet light further comprises a radiation intensity between 50mJ/cm² and 200 mJ/cm², a radiation temperature between 150° C. and 250°C., and a radiation time between 10 and 60 minutes.
 5. The method ofclaim 2, wherein exposure to the polymeric cross-linking inducingradiant energy is carried out prior to the first or second hard bakestep comprising a thermal heating step.
 6. The method of claim 2,wherein exposure to the polymeric cross-linking inducing radiant energyis carried out at least during a portion of the first or second hardbake process.
 7. The method of claim 1, wherein the first or second hardbake process comprises a baking temperature of from about 250° C. toabout 350° C.
 8. The method of claim 1, wherein the first smaller volumeis compared to the final resist portion volume by about 5% to about 50%.9. The method of claim 1, further comprising the step of removing resistcomprising the final resist portion according to at least one of anashing process and a wet stripping process to form a free-standingstructural member.
 10. The method of claim 1, wherein the structuralmaterial is selected from the group consisting of metals, nitrides,oxides, carbides, and titanates.
 11. The method of claim 1, wherein thestructural material is selected from the group consisting of metals,metal nitrides, refractory metals, refractory metal nitrides, oxides,carbides, and piezo-electric oxides.
 12. A method for forming a freestanding micro-structural member comprising the steps of: providing asubstrate; forming a first sacrificial resist layer over the substrate;patterning the first sacrificial resist layer to form a first resistportion; subjecting the first resist portion to at least a first posttreatment process to form the first resist portion having a firstvolume; forming at least a second sacrificial resist layer on a topsurface of said first resist portion followed by patterning andconducting at least a second post treatment process to form a finalresist portion having a final volume; depositing at least one structuralmaterial layer over the final resist portion; and removing said finalresist portion.
 13. A method for forming a free standingmicro-structural member over a resist portion with improved dimensionaltolerances comprising the steps of: providing a substrate; forming afirst resist layer over the substrate; patterning the first resist layerto form a first resist portion having a predetermined first volumesmaller compared to a predetermined final resist portion volume;subjecting the first resist portion to a first curing process comprisingdeep UV irradiation and thermal heating for a predetermined period toharden the first resist portion; forming at least a second resist layerhaving a predetermined thickness over the first resist portion followedby patterning and a second curing process to form the final resistportion volume; depositing at least one structural material layer overthe final resist portion; and, removing the final resist portionaccording to at least one of an ashing and a wet stripping process toform a free standing structural member.
 14. The method of claim 13,wherein the first and second curing processes comprise exposure to adeep UV irradiation prior to a thermal heating period.
 15. The method ofclaim 13, wherein the thermal heating period comprises a temperature offrom about 250° C. to about 350° C.
 16. The method of claim 14, whereinthe first and second curing processes comprise exposure to a deep UVirradiation during at least a portion of a thermal heating period. 17.The method of claim 13, wherein the first volume is smaller compared tothe final resist portion volume by about 5% to about 50%.
 18. The methodof claim 13, wherein the first volume is smaller compared to the finalresist portion volume from about 10% to about 33%.
 19. The method ofclaim 13, wherein the first volume comprises sidewall portions formedhaving a smaller dimension by a factor of about ½ compared to a smallerthickness dimension.
 20. The method of claim 13, wherein the structuralmaterial is selected from the group consisting of metals, metalnitrides, refractory metals, refractory metal nitrides, oxides,carbides, and metal titanates.